The present invention generally relates to an electrochemical cell construction. More particularly, the present invention relates to the containers and collector assemblies used for an electrochemical cell, such as an alkaline cell.
FIG. 1 shows the construction of a conventional C sized alkaline cell 10. As shown, cell 10 includes a cylindrically-shaped can 12 having an open end and a closed end. Can 12 is preferably formed of an electrically conductive material, such that an outer cover 11 welded to a bottom surface 14 at the closed end of can 12 serves as an electrical contact terminal for the cell.
Cell 10 further typically includes a first electrode material 15, which may serve as the positive electrode (also known as a cathode). The first electrode material 15 may be preformed and inserted into can 12, or may be molded in place so as to contact the inner surfaces of the can 12. For an alkaline cell, first electrode material 15 will typically include MnO.sub.2. After the first electrode 15 has been provided in can 12, a separator 17 is inserted into the space defined by first electrode 15. Separator 17 is preferably a non-woven fabric. Separator 17 is provided to maintain a physical separation of the first electrode material 15 and mixture of electrolyte and a second electrode material 20 while allowing the transport of ions between the electrode materials.
Once separator 17 is in place within the cavity defined by first electrode 15, an electrolyte is dispensed into the space defined by separator 17, along with the mixture 20 of electrolyte and a second electrode material, which may be the negative electrode (also known as the anode). The electrolyte/second electrode mixture 20 preferably includes a gelling agent. For a typical alkaline cell, mixture 20 is formed of a mixture of an aqueous KOH electrolyte and zinc, which serves as the second electrode material. Water and additional additives may also be included in mixture 20.
Once the first electrode 15, separator 17, the electrolyte, and mixture 20 have been formed inside can 12, a preassembled collector assembly 25 is inserted into the open end of can 12. Can 12 is typically slightly tapered at its open end. This taper serves to support the collector assembly in a desired orientation prior to securing it in place. After collector assembly 25 has been inserted, an outer cover 45 is placed over collector assembly 25. Collector assembly 25 is secured in place by radially squeezing the can against collector assembly 25. The outer cover 45 is then placed over and in contact with collector assembly 25. The end edge 13 of can 12 is then crimped over the peripheral lip of collector assembly 25, thereby securing outer cover 45 and collector assembly 25 within the end of can 12. As described further below, one function served by collector assembly 25 is to provide for a second external electrical contact for the electrochemical cell. Additionally, collector assembly 25 must seal the open end of can 12 to prevent the electrochemical materials therein from leaking from this cell. Additionally, collector assembly 25 must exhibit sufficient strength to withstand the physical abuse to which batteries are typically exposed. Also, because electrochemical cells may produce hydrogen gas, collector assembly 25 may allow internally-generated hydrogen gas to permeate therethrough to escape to the exterior of the electrochemical cell. Further, collector assembly 25 should include some form of pressure relief mechanism to relieve pressure produced internally within the cell should this pressure become excessive. Such conditions may occur when the electrochemical cell internally generates hydrogen gas at a rate that exceeds that at which the internally-generated hydrogen gas can permeate through the collector assembly to the exterior of the cell.
The collector assembly 25 shown in FIG. 1 includes a seal 30, a collector nail 40, an inner cover 44, a washer 50, and a plurality of spurs 52. Seal 30 is shown as including a central hub 32 having a hole through which collector nail 40 is inserted. Seal 30 further includes a V-shaped portion 34 that may contact an upper surface 16 of first electrode 15.
Seal 30 also includes a peripheral upstanding wall 36 that extends upward along the periphery of seal 30 in an annular fashion. Peripheral upstanding wall 36 not only serves as a seal between the interface of collector assembly 25 and can 12, but also serves as an electrical insulator for preventing an electrical short from occurring between the positive can and negative contact terminal of the cell.
Inner cover 44, which is formed of a rigid metal, is provided to increase the rigidity and supports the radial compression of collector assembly 25 thereby improving the sealing effectiveness. As shown in FIG. 1, inner cover 44 is configured to contact central hub portion 32 and peripheral upstanding wall 36. By configuring collector assembly 25 in this fashion, inner cover 44 serves to enable compression of central hub portion 32 by collector nail 40 while also supporting compression of peripheral upstanding wall 36 by the inner surface of can 12.
Outer cover 45 is typically made of a nickel-plated steel and is configured to extend from a region defined by the annular peripheral upstanding wall 36 of seal 30 and to be in electrical contact with a head portion 42 of collector nail 40. Outer cover 45 may be welded to head portion 42 of collector nail 40 to prevent any loss of contact. As shown in FIG. 1, when collector assembly 25 is inserted into the open end of can 12, collector nail 40 penetrates deeply within the electrolyte/second electrode mixture 20 to establish sufficient electrical contact therewith. In the example shown in FIG. 1, outer cover 45 includes a peripheral lip 47 that extends upwardly along the circumference of outer cover 45. By forming peripheral upstanding wall 36 of seal 30 of a length greater than that of peripheral lip 47, a portion of peripheral upstanding wall 36 may be folded over peripheral lip 47 during the crimping process so as to prevent any portion of the upper edge 13 of can 12 from coming into contact with outer cover 45.
Seal 30 is preferably formed of nylon. In the configuration shown in FIG. 1, a pressure relief mechanism is provided for enabling the relief of internal pressure when such pressure becomes excessive. Further, inner cover 44 and outer cover 45 are typically provided with apertures 43 that allow the hydrogen gas to escape to the exterior of cell 10. The mechanism shown includes an annular metal washer 50 and a plurality of spurs 52 that are provided between seal 30 and inner cover 44. The plurality of spurs 52 each include a pointed end 53 that is pressed against a thin intermediate portion 38 of seal 30. Spurs 52 are biased against the lower inner surface of inner cover 44 such that when the internal pressure of cell 10 increases and seal 30 consequently becomes deformed by pressing upward toward inner cover 44, the pointed ends 53 of spurs 52 penetrate through the thin intermediate portion 38 of seal 30 thereby rupturing seal 30 and allowing the escape of the internally-generated gas through apertures 43.
Although the above-described collector assembly 25 performs all the above-noted desirable functions satisfactorily, as apparent from its cross-sectional profile, this particular collector assembly occupies a significant amount of space within the interior of the cell 10. Because the exterior dimensions of the electrochemical cell are generally fixed by the American National Standards Institute (ANSI), the greater the space occupied by the collector assembly, the less space that there is available within the cell for the electrochemical materials. Consequently, a reduction in the amount of electrochemical materials that may be provided within the cell results in a shorter service life for the cell. It is therefore desirable to maximize the interior volume within an electrochemical cell that is available for the electrochemically active components.
It should be noted that the construction shown in FIG. 1 is but one example of a cell construction. Other collector assemblies exist that may have lower profiles and hence occupy less space within the cell. However, such collector assemblies typically achieve this reduction in occupied volume at the expense of the sealing characteristics of the collector assembly or the performance and reliability of the pressure relief mechanism. It is therefore desirable to construct an electrochemical cell where the space occupied by the collector assembly and the space occupied by the container volume are minimized while still maintaining adequate sealing characteristics and a reliable pressure relief mechanism.
The measured external and internal volumes for several batteries that were commercially available as of the filing date of this application are listed in the tables shown in FIGS. 2A and 2B. The tables list the volumes (cc) for D, C, AA, and AAA sized batteries. Also provided in FIG. 2A is a percentage of the total cell volume that constitutes the internal volume that is available for containing the electrochemically active materials. The total cell volume includes all of the volume, including any internal void spaces, of the battery. For the battery shown in FIG. 1, the total volume ideally includes all of the cross-hatched area as shown in FIG. 3A. The "internal volume" of the battery is represented by the cross-hatched area shown in FIG. 3B. The "internal volume," as used herein, is that volume inside the cell or battery that contains the electrochemically active materials as well as any voids and chemically inert materials (other than the collector nail) that are confined within the sealed volume of the cell. Such chemically inert materials may include separators, conductors, and any inert additives in the electrodes. As described herein, the term "electrochemically active materials" includes the positive and negative electrodes and the electrolyte.
The collector assembly volume includes the collector nail, seal, inner cover, and any void volume between the bottom surface of the negative cover and the seal (indicated by the cross-hatched area in FIG. 3C). It should be appreciated that the sum total of the "internal volume," "collector assembly volume," and "container volume" is equal to the total volume. Accordingly, the internal volume available for electrochemically active materials can be confirmed by measuring the collector assembly volume and container volume and subtracting the collector assembly volume and the container volume from the measured total volume of the battery. The "container volume" includes the volume of the can, label, negative cover, void volume between the label and negative cover, positive cover, and void volume between the positive cover and can (shown by the cross-hatched area in FIG. 3D). If the label extends onto and into contact with the negative cover, the void volume present between the label and negative cover is included in the container volume, and therefore is also considered as part of the total volume. Otherwise, that void volume is not included in either of the container volume or the total volume. The collector assembly volume and the percentage of the total cell volume that constitutes the collector assembly volume is provided in FIG. 2B for those commercially available batteries listed in FIG. 2A.
The total battery volume, collector assembly volume, and internal volume available for electrochemically active material for each battery are determined by viewing a Computer Aided Design (CAD) drawing, a photograph, or an actual cross section of the battery which has been encased in epoxy and longitudinally cross-sectioned. The use of a CAD drawing, photograph, or actual longitudinal cross section to view and measure battery dimensions allows for inclusion of all void volumes that might be present in the battery. To measure the total battery volume, the cross-sectional view of the battery taken through its central longitudinal axis of symmetry is viewed and the entire volume is measured by geometric computation. To measure the internal volume available for electrochemically active materials, the cross-sectional view of the battery taken through its central longitudinal axis of symmetry is viewed, and the components making up the internal volume, which includes the electrochemically active materials, void volumes and chemically inert materials (other than the collector nail) that are confined within the sealed volume of the cell, are measured by geometric computation. Likewise, to determine volume of the collector assembly, the cross-sectional view of the battery taken through its central longitudinal axis of symmetry thereof is viewed, and the components making up the collector assembly volume, which include the collector nail, seal, inner cover, and any void volume defined between the bottom surface of the negative cover and the seal, are measured by geometric computation. The container volume may likewise be measured by viewing the central longitudinal cross section of the battery and computing the volume consumed by the can, label, negative cover, void volume between the label and negative cover, positive cover, and void volume between the positive cover and the can.
The volume measurements are made by viewing a cross section of the battery taken through its longitudinal axis of symmetry. This provides for an accurate volume measurement, since the battery and its components are usually axial symmetric. To obtain a geometric view of the cross section of a battery, the battery was first potted in epoxy and, after the epoxy solidified, the potted battery and its components were ground down to the central cross section through the axis of symmetry. More particularly, the battery was first potted in epoxy and then ground short of the central cross section. Next, all internal components such as the anode, cathode, and separator paper were removed in order to better enable measurement of the finished cross section. The potted battery was then cleaned of any remaining debris, was air dried, and the remaining void volumes were filled with epoxy to give the battery some integrity before completing the grinding and polishing to its center. The battery was again ground and polished until finished to its central cross section, was thereafter traced into a drawing, and the volumes measured therefrom.
Prior to potting the battery in epoxy, battery measurements were taken with calipers to measure the overall height, the crimp height, and the outside diameter at the top, bottom, and center of the battery. In addition, an identical battery was disassembled and the components thereof were measured. These measurements of components of the disassembled battery include the diameter of the current collector nail, the length of the current collector nail, the length of the current collector nail to the negative cover, and the outside diameter of the top, bottom, and center of the battery without the label present.
Once the battery was completely potted in epoxy and ground to center through the longitudinal axis of symmetry, the cross-sectional view of the battery was used to make a drawing. A Mitutoyo optical comparitor with QC-4000 software was used to trace the contour of the battery and its individual components to generate a drawing of the central cross section of the battery. In doing so, the battery was securely fixed in place and the contour of the battery parts were saved in a format that could later be used in solid modeling software to calculate the battery volumes of interest. However, before any volume measurements were taken, the drawing may be adjusted to compensate for any battery components that are not aligned exactly through the center of the battery. This may be accomplished by using the measurements that were taken from the battery before cross sectioning the battery and those measurements taken from the disassembled identical battery. For example, the diameter and length of the current collector nail, and overall outside diameter of the battery can be modified to profile the drawing more accurately by adjusting the drawing to include the corresponding known cross-sectional dimensions to make the drawing more accurate for volume measurements. The detail of the seal, cover, and crimp areas were used as they were drawn on the optical comparitor.
To calculate the volume measurements, the drawing was imported into solid modeling software. A solid three-dimensional volume representation was generated by rotating the contour of the cross section on both the left and right sides by one-hundred-eighty degrees (180.degree.) about the longitudinal axis of symmetry. Accordingly, the volume of each region of interest is calculated by the software and, by rotating the left and right sides by one-hundred-eighty degees (180.degree.) and summing the left and right volumes together an average volume value is determined, which may be advantageous in those situations where the battery has non-symmetrical features. The volumes which include any non-symmetrical features can be adjusted as necessary to obtain more accurate volume measurements.